With the development of hybridization techniques for the production of antibody-producing cells it is now possible to obtain virtually unlimited quantities of monoclonal antibodies directed against defined antigens. During the past year we have established twenty independent hybrid cell lines that produce monoclonal antibodies against antigens of leukemia viruses. The purpose of this proposed study is to evaluate the use of these monoclonal antibodies for the therapy of neoplastic disease in mice. Specifically, we will continue to isolate a series of hybrid cells that are producing monoclonal antibodies against the proteins of mouse leukemia virus and leukemia-associated antigens. Therapeutic potential of these antibodies in the treatment of transplanted and spontaneous leukemias and sarcomas will then be evaluated. The class of immunoglobulin and specificity of the monoclonal antibodies, as well as their interactions with host factors (complement components, lymphocytes, macrophages) in the tumor rejection phenomena, will be determined.